Tungsten, generally in the form of tungsten oxide, WO.sub.3, is useful as a component of a variety of catalysts for chemical reactions. In most cases, in the manufacture of these catalysts it is necessary at some stage to provide a water soluble tungsten compound from which the tungsten oxide is derived. For example, certain types of catalysts comprise porous carriers such as, for example, alumina impregnated with compounds of catalytic elements such as tungsten. In the preparation of such catalysts the carrier materials are immersed, or slurried, in solutions of the compounds of the catalytic elements and then dried, leaving the catalyst compound intimately associated with the carrier. Ammonium metatungstate, (NH.sub.4).sub.6 H.sub.2 W.sub.12 O.sub.40.xH.sub.2 O is a particularly useful solute in such solutions. It offers the advantage of being readily soluble in water and, in addition, it decomposes readily on heating to leave the desired catalytic residue of tungsten oxide in the carrier. Alkali metal salts of the tungstates are also water soluble, but their use in catalyst manufacture is restricted by the fact that the presence of residual alkali metals frequently is objectionable in catalytic materials.
It has been proposed to obtain ammonium metatungstate solutions by baking crystalline ammonium paratungstate at temperatures of the order of 500.degree. F. to drive off ammonia and to then dissolve the product of the baking operation in water. As is well known, a certain amount of ammonium metatungstate is produced by such a baking operation. However, thermogravimetric studies of this process have demonstrated that the ammomium paratungstate is not converted entirely to ammonium metatungstate at any particular temperature, nor after any particular baking time. To the contrary, other degradation products, particularly insoluble tungsten oxide also are continuously formed during the baking. In addition, certain amounts of unconverted ammonium paratungstate remain. As a result, substantial amounts of insoluble tungsten oxide and also ammonium paratungstate, which is only slightly soluble, remain undissolved when it is attempted to dissolve the product of the baking operation making it necessary to filter the solution before use, and discard or reprocess the insoluble materials. Also, as indicated above, the small amount of paratungstate remaining in solution my complicate the use of the solution because of its tendency to precipitate out with temperature changes or evaporation of the water.
U.S. Pat. No. 3,591,331 discloses a process for producing ammonium metatungstate without an appreciable formation of ammonium paratungstate. The process comprises contacting an aqueous ammonmium tungstate solution having a pH of at least about 9 with an organic extractant solution comprising di-2-ethylhexyl phosphoric acid, and a water insoluble hydrocarbon solvent, the components being in specified ratios to extract ammonium ions from the aqueous solution, separating the resultant aqueous solution from the organic solution, heating the aqueous solution to a temperature of at least about 60.degree. C. for at least about 1 hour and recovering an essentially pure ammonium metatungstate.
U.S. Pat. No. 3,857,928 discloses a process for producing crystalline ammonium metatungstate from ammonium tungstate solution by introducing the ammonium tungstate solution into an ion exchange column containing a weak acid (carboxylic group) cation exchange resin, and collecting the effluent from the column until a pH of about 3.5 is reached. The effluent solution is then digested at about 98.degree. C. for about 5 hours, followed by crystallization of ammonium metatungstate by conventional processes, such as, evaporation or spray drying.
U.S. Pat. No. 3,857,929 discloses a process for producing crystalline ammonium metatungstate from ammonium tungstate solution by introducing a strong acid cation exchange resin containing sulphonic group batchwise into an ammonium tungstate solution until a pH of about 3.5 is reached. The resin is then removed by filtration and the filtered solution is digested at about 98.degree. C. for at least about 5 hours, followed by crystallization of ammonium metatungstate by conventional processes, such as, evaporation or spray drying.
U.S. Pat. No. 3,956,474 discloses a process for producing ammonium metatungstate from ammonium tungstate which involves the addition of about 3.6 percent by weight of silica to an ammonium tungstate solution, digestion for at least about 4 hours at a temperature of at least about 98.degree. C. followed by filtration to remove silica from the ammonium metatungstate solution. Typically about 0.4 percent by weight of silica remains after filtering. The resulting ammonium metatungstate solution may be further processed to recover solid ammonium metatungstate, such as by evaporation or spray drying.
U.S. Pat. No. 3,936,362 discloses a process for producing ammonium metatungstate and other species by passing tungstate ions through an anion exchange membrane into an aqueous solution containing ammonium cations under the driving force of an electrical potential for a time sufficient to achieve a pH within the range in which the desired tungsten compound will form.
U.S. Pat. No. 3,175,881 discloses a method for producing crystalline ammonium metatungstate from ammonium paratungstate to the point at which a slurry containing about 9% by weight of material exhibits a pH of from about 3 to about 5, the heating time being generally from about 4 to about 8 hours, followed by evaporation of the slurry to about one-third of its original volume, filtering the concentrated slurry and then crystallizing ammonium metatungstate. Based on the WO.sub.3 contents of the ammonium paratungstate starting material and the ammonium metatungstate produced, the yield is about 78.1%.
A more effective method of producing crystalline ammonium metatungstate from ammonium paratungstate in higher yields than the above would be desirable.